Rename model based variable names to entity (#24198)

## Context
While looking through the client crate, I noticed that some of the old
functions and variables were still using gpui::model name that was
deprecated during the gpui3 transition. This PR renames those instances
of model to entity to be more inline with gpui3.

In addition, I also renamed `model` to `entity` in cases found by the
below search terms given by @someone13574

- model = cx.
- model: Entity
- model: &Entity
- OpenedModelHandle
- model.update
- model.upgrade
- model = .*\.root (regex)
- parent_model
- model = cx.new
- cx.spawn(move |model

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -849,10 +849,10 @@ impl TestClient {
     ) -> (Entity<Workspace>, &'a mut VisualTestContext) {
         let window = cx.update(|cx| cx.active_window().unwrap().downcast::<Workspace>().unwrap());
 
-        let model = window.root(cx).unwrap();
+        let entity = window.root(cx).unwrap();
         let cx = VisualTestContext::from_window(*window.deref(), cx).as_mut();
         // it might be nice to try and cleanup these at the end of each test.
-        (model, cx)
+        (entity, cx)
     }
 }
 
@@ -861,9 +861,9 @@ pub fn open_channel_notes(
     cx: &mut VisualTestContext,
 ) -> Task<anyhow::Result<Entity<ChannelView>>> {
     let window = cx.update(|_, cx| cx.active_window().unwrap().downcast::<Workspace>().unwrap());
-    let model = window.root(cx).unwrap();
+    let entity = window.root(cx).unwrap();
 
-    cx.update(|window, cx| ChannelView::open(channel_id, None, model.clone(), window, cx))
+    cx.update(|window, cx| ChannelView::open(channel_id, None, entity.clone(), window, cx))
 }
 
 impl Drop for TestClient {